January 27, 2011

Check out NASA’s Astro-venture program for students! An interactive game called Design a Planet lets you build an alien planet habitable for human life. Can you build a habitable planet around a low-mass red dwarf star?

January 25, 2011

Areios orbits Hemera somewhere between where Mercury and Venus would be in our Solar System; at first, Areios lies just within its habitability zone where liquid water could exist on the surface, and because Hemera initially output less light when Areios was forming, it seems unlikely that life could arise on this frigid planet right from the start. On Earth, life may have formed just as soon as the crust solidified and the oceans condensed from the atmosphere, yet our planet would have been cooler than it is now were it not for the greenhouse effect. This early arrival for life suggests that life could be common in the universe if it can be spawned on a habitable planet so early in its formation. Because Areios is on the edge of the habitable zone, it may take some time to warm before it can be habitable for life. The habitability zone for life will change over time as Hemera evolves; Areios is positioned in the very outer habitable zone near the beginning of Hemera’s life and by the end of its main sequence stage, Areios is only just tucked inside the inner edge of that habitable zone. For the 38 billion years or so that Hemera is in the main sequence stage, Areios is within the habitable zone for 30 billion of those years. For the first and last four billion years of Hemera’s evolution, Areios will be sterilized of all life, first because of freezing temperatures early on and then because of the boiling temperatures near the end of Hemera’s life. As Areios’ surface temperature gets pushed hotter, Areios will have to shed more and more of its thick atmosphere like a jacket to cool off until its atmosphere is too thin to support liquid water on its surface and the oceans boil away. But more on the evolution of a habitable planet later…

Areios formed from the collision of planetesimals billions of years ago; these violent interactions also created two out of three of its moons and a third moon was captured later during a period of asteroid and comet bombardment. When the cataclysms of the planet formation ended, Areios was a still-molten ball of rock with a swirling ring of debris that would later become its moons. Areios is more massive than Earth and contains a bigger mantle and a thinner crust. The importance of this will be revealed later on, but this distinction is not trivial when it comes to the potential for life on Areios. Because of Areios’ girth, the planet would take longer to cool and the internal portions of the planet would stay hotter for longer because the core is wrapped in a much thicker insulating blanket of mantle. This early planet would soon cool on the outside, though, and a process called differentiation would occur; Areios was at first a well-mixed sphere or magma, but it began to cool and settle. The crust formed like a skin like a bowl of soup left to cool; this outermost layer is only a few kilometers thick, but covers the entire planet and serves as the palette for the thin veneer of life that is to come. The crust covers the mantle of the planet, which makes up the bulk of Areios’ mass. The mantle is a made of melted rock kept solid by the intense pressure coming down on it; unlike Jules Verne’s Journey to the Center of the Earth, there are no caverns or caves in the mantle because this solid rock can flow like a liquid and would quickly fill any void beneath the planet, despite the fact that pressures make the molten material behave like solid rock.

This is a depiction of what a planet like Areios would look like early in its formation.

As Areios formed, there were three processes that kept generating the internal heat of the mantle; the kinetic energy of impacts, differentiation, and radiogenic heating. During the formation of Earth, there was a period called the Late Heavy Bombardment where more comet and asteroid impacts struck the planet, boiling the oceans and melting the crust for a couple hundred million years before the rain of fire subsided. When these objects struck the Earth, their gravitational potential energy as they fell to the earth was converted to kinetic energy in the form of heat. Areios experienced a similar event to the late heavy bombardment for a couple of hundred million years after the planet formed and for a while the kinetic energy from those impacts kept heating the planet, but once those impacts subsided, the planet cooled enough to form crust and the oceans. Areios started out as a homogeneous ball of magma, but slowly the heavier metals started to settle in the core of the planet. As these denser materials sank into the mantle, their potential energy was converted into kinetic energy until the planet differentiated into the three distinct layers of the crust, mantle and core. Once these three layers were fully formed, the planet no longer generated heat by differentiation. The final and only ongoing way the interior of the planet generates heat is through radiogenic heating. When the planet formed, it incorporated some heavier unstable elements like thorium and uranium. Over time, these elements would decay into lighter elements like potassium or lead; this radioactive decay would release energy in the form of heat that keeps the internal parts of Areios hot. This Late Heavy Bombardment era for Areios would deliver water to the planet and later determined how much of Areios will be covered in lakes and oceans.

January 17, 2011

Hemera’s solar system formed when a molecular gas cloud called a nebula compacted into its center and formed a denser mass of dust. As material got attracted to the center of this cloud, it released heat through transforming gravity’s potential energy into the kinetic energy of motion. When gravitational forces began the collapse, the cloud’s slower rotation picked up under the conservation of angular momentum. As the center of this cloud started to attract more mass, the gravitational pull that this mass became stronger and the mass was able to pull into more mass and release more energy by this transformation. This process went on until the gravity of this gas ball became so massive that it started to crush the hydrogen in the center until it has dense enough and hot enough to form helium. Once this object was hot enough to undergo nuclear fusion of hydrogen into helium, it became a main sequence star.

As the solar system was forming, grains of rock and gas were swirling in an orbit around Hemera; these bits would collide frequently and sometimes these particles would smash into each other and form larger particulates. Eventually, these collisions would cause the orbits of all of the particles to roughly take on an average speed and direction, so the solar system would flatten out into a plane where all of the chunks of planetesimals would orbit around in. Eventually, those tiny grains of dust accreted into large rocky protoplanets, satellites ranging from the size of a continent to roughly the size of Mars. These planetesimals would collide with one another and form the planets we see now. Areios experienced one extraordinary collision that formed two of its three moons. A collision with a mercury-sized object called Dione created two silica-rich moons that orbit around the planet. The impact sent over 100 moonlet fragments into orbit around the planet and eventually formed three moons; one of them was cast out into deep space after six weeks. These two remaining moons, Otus and Ephialtes, are revolving farther and farther from the planet each year and this slows down the orbit of Areios over billions of years. Eventually, both of these moons will escape Areios’ gravitational pull and roll off into space. A third moon Eriboea was snagged in a later capture event that nestled it in between the first and third moons created by the impact event. Spiraling inward retrograde to the first and third moons, this moon will one day collide with Areios’ Roche limit and be ground up in a planetary ring. When that debris gets dislodged by gravity and some rains down on the planet, it would spell doom for anything unlucky enough to be alive at the time.

In Hemera’s solar system, we see a similar process of accretion that leads to two planets being formed; one is a massive gas planet that orbits far out into the Solar System; Alkyoneus, more massive than Jupiter, captured seven planetesimals into a lunar orbit before they could be smashed in a collision. These seven moons called the Alkynonides would large enough to be planets with satellites of their own, were they not captured by the gas giant’s formidable gravity. These planetesimals have plate tectonics, volcanism and a thin atmosphere, but orbit too far from the Sun to have liquid water on their surface. The radiation blasted from the gas giant irradiates anything on the surface, making it difficult for life to start. With no oceans or thick atmosphere to protect life from the harmful radiation, and it seems unlikely that life would spawn around Alkyoneus. The Alkyonides are in a part of deep space that’s far from the habitable zone where liquid water can exist on the surface of a planet. But liquid water could exist on these moons underneath the crust, warmed by the volcanism of the internal heat generated within the planet.

Our final stop is a tiny speck out in the far reaches of the solar system; this lonely planet is much like our Pluto, covered in a layer of ice along with its own little frozen satellite. Perses and the satellite Hekate were once part of the Alkyonides, but early on in the formation of the solar system, Perses and its satellite were dislodged from orbit and were lost in space until they finally reached a stable orbit at the very fringes of the solar system. Perses was the smallest of the Alkyonides, but still managed to snare a comet for a moon; when it was flung into a orbit at the outer regions of space, a comet settled in an orbit around Perses. Although Perses was once tectonically and volcanically active, it no longer produces a magnetic field or has any significant atmosphere. Now it just hangs out in the dead of space, the last stop in the solar system before the great beyond of the stars.

January 10, 2011

We’re going to focus on a star that’s just within this galactic habitable zone; it’s on the outer edge of the galaxy and is too dim to see from the center of the galactic habitable zone. Our star is called Hemera and it is even punier than our mediocre Sun; while the Sun fuses lighter elements into calcium and gives off a healthy yellow light Hemera is smaller, less compact and looks feverish with its deep orange-red glow. This hue is because Hemera has less mass than our star so gravity doesn’t push down on it as hard, and it’s less dense than the Sun because it burns its fuel more slowly, and with less luminosity. This means that our star will stay in the main sequence stage of its life for longer than the Sun, which is good news for our creatures on this planet orbiting Hemera; it gives them more time to evolve into a form that could one day jump ship before Hemera goes nova.

For our star with a mass of about three-fourths the mass of the Sun, life could potentially live on Areios around Hemera as a main sequence star for about 30.5 billion years before Hemera will go defunct. At 70% of our Sun’s mass, it would only shine at about a quarter of the brightness of our Sun.24 This could be problematic for life if the planet isn’t situated close to its star to stay warm. Once a planet gets too close to its star, though, it may become tidally-locked, like our moon is to the Earth, with the same side perennially pointing towards the surface of Hemera. One side of the world would boil and the other side would freeze. A planet can overcome this with an atmosphere thick enough to circulate heat and keep one side of the planet’s atmosphere from boiling and the other side from freezing.7 Curiously, the planet would be divided into three distinct zones; one of perpetual light, one of perpetual darkness, and one of perpetual twilight between the two opposing hemispheres of light and dark. 6 Astrobiologist Nancy Kiang suspects that plants on this world would be jet black to absorb as much of the dim sunlight as possible around a red dwarf star.8

Note that the Sun-like Star on the right is the most moassive and has the widest habitable zone of the three stars.

For a planet orbiting a red dwarf star, the only way to keep warm is to orbit in this tidally-locked configuration that would keep one side of the planet always pointing to the star and one side always pointing away, but a new model of planet habitability suggests that planets orbiting a red dwarf may be host to habitable planets. Red dwarf stars can exist in the main sequence stage for tens of billions or maybe even a 100 billion years, certainly longer than the age of the universe so far.5 This would mean that a red dwarf star could be habitable to life for tens of billions of years, much longer than on Earth or Areios. Also, because these stars don’t burn out at the rate of more massive stars, they make up a greater percentage of the stars in the night sky. Astronomers estimate as many as 75% percent of the stars in the universe could be red dwarf stars.10 This makes planets within the habitability zone of red dwarves a priority for astronomers looking for habitable planets, if only technology were sensitive enough to detect such dim stars and their planets. However, there are an abundance of them for planet hunters to find.

It should be mentioned that a star will get brighter with age. Over the lifetime of a star, the luminosity increases as mass decreases and gravity pushes down on the star with more intensity. As a star loses fuel, gravity pushes it harder towards a center point which concentrates the remaining fuel it has left into a more efficient sphere that burns harder to correct for the now overpowering gravity to bring the star back to equilibrium. When stars are just forming, they release high-energy radiation that would sterilize the surface of any planet too close; this activity lessens with time, but it would leave the surface of a planet uninhabitable at first. Low-mass red dwarf stars undergo these shifts in luminosity called flares to a much greater extent than a star like Hemera.9 Once Hemera gets over this early phase, it will produce less ultraviolet radiation than our sun, and generate more infrared and visible light, making it potentially safer for life on the surface. Our Sun has grown about 33% brighter over the last 4.5 billion years.3 At first Areios may be less habitable for life because it’s so cold orbiting around a dim star, but over time that star would get hotter and brighter.

The surface temperature of a planet isn’t solely dependant on the amount of radiation it receives from its star, either. Surface temperature also depends on how effectively the atmosphere can trap heat in a phenomena called the greenhouse effect. Without the greenhouse effect, Earth would be much cooler and while carbon dioxide levels have fluctuated over geologic time, there is ubiquitous consent that rising carbon dioxide levels are causing a more pronounced greenhouse effect. The greenhouse effect occurs when certain greenhouse gases like water vapor, carbon dioxide, and methane allow shorter wavelength light to pass through the atmosphere, but traps longer wavelength infrared light, to keep it from escaping.1 Infrared light heats up the planet enough on Earth to keep the tropics from freezing, but when humans pumped more carbon dioxide into the atmosphere from burning fossil fuels, it trapped more heat and caused a series of climate changes that we have yet to figure out what the long-term impact of that decision will truly be. The same mechanism that intervenes to maximize the surface temperature on our planet wouldn’t be as pronounced on a world like Areios with a hyperactive plate tectonic system to scrub carbon dioxide from the planet. The greenhouse effect that is causing widespread climate change on Earth could keep Areios warm enough to form liquid water on the surface.

January 3, 2011

The most distant object we’ve sent through our solar system is the Voyager 1 probe. Launched on September 5, 1977 it has since reached the very edge of our solar system and will soon pass out of the heliopause, or the point where our sun’s solar wind is weaker than the interstellar wind of other stars, effectively drawing the end of our solar system. Well after its original mission was completed, Voyager 1 continues to collect data on our Sun and its perplexing heliosphere.1 After traveling for 33 years, this probe has only made it 0.002 light years from our planet, but it has flown by everything in our solar system. It’s from this perspective that we begin our story of an alien solar system; from the outgoing point of view of an interstellar probe, the star we are about to discuss looks no bigger or brighter than any other star in the night sky, (and it’s not even visible to the naked eye) but it holds the little-known distinction of an abode for life.

Stars can be classified by their brightness, their size, their composition, and their ability to undergo fusion. Scientists also classify stars based on their spectral characteristics, or what color the star looks like as it burns, which is based on what kinds of heavier elements are getting fused in the star’s interior, which in turn is determined by how hot the star is. We can graph these stars in a diagram that compares the mass of a star with its luminosity. This is called the Hertzsprung-Russell diagram and it shows a band of stars on in the center sloping down and to the right that signifies the stability strip of main sequence stars.2 This strip shows the correlation between the mass of a star and the brightness and temperature with which it burns. Stars generate their heat and light through the process of fusing elements together to form heavier elements called nucleosynthesis. A star maintains its integrity by balancing the inward push of gravity with the outward thrust of nuclear fusion. When the star runs out of fuel to continue fusion, the inward push of gravity overcomes the weakened thrust of fusion and the star collapses, getting hotter as the gravitational collapse causes potential energy to be released, concentrating the heat of the star into a much smaller volume. Eventually, that star releases all of that energy in a nova.3 In our star, mostly hydrogen fuses together under immense temperature and pressure to form helium; in other stars hotter than our Sun, heavier elements can be formed in greater quantities and at a faster rate than our scrawny little star. Stars are the furnaces that create all the chemical elements in the universe and when these gas balls burn out and explode in a nova, releasing the heavy elements that will one day make up the satellites around another solar system. The more massive the star, the bigger the explosion and these supernovas release every known element out into the universe.

When the universe began, there was only hydrogen and helium with a smattering of lithium and beryllium in the smallest trace amounts. The first generation planets to form in the afterglow of the Big Bang were only made of hydrogen and helium, more akin to the gas giants in Earth’s solar system rather than a terrestrial planet like Areios. It wasn’t until the first generation stars went supernova that we see any satellite resembling a rocky planet because heavier metals that make up the bulk of our world weren’t forged until a supernova explosion sent those products out into the stars. The second generation planets could have been made of rocky material and may have been habitable for life once elements like carbon, nitrogen, and oxygen were created. Areas of a galaxy with more hydrogen and helium tend to create more bigger stars that create more violent supernova explosions.

Because of this, NGC 772 has areas of more star-building compared to other areas with less star-building and this means that certain regions of NGC 772 is more habitable than other regions, which is conceptualized by the galactic habitable zone.4 The GHZ of NGC 772 looks like a donut; the very center of our galaxy is too close to a super massive black hole at the center of the galaxy; that black hole will spit out dangerous radiation and any planet too close would get irradiated with high-energy particles that could tear apart a cell. Any planet too close to the center would be sterilized. Any star in the center of the galaxy would also be closer to more massive stars and would be subjected to more devastating supernova explosions that might hinder the development of life. Too far out into the edge of the galaxy and there’s not enough gas to form stars and planets and any satellite out there would not have enough metal to build a habitable planet. It’s somewhere in between the outer and inner portions of our galaxy that we find the right space conducive for building habitable planet and stars. This concept of a “goldilocks” zone that’s just right for life will come up again later in our discussion a planet’s orbit around a star. But more on that later…